1. Field of the Invention
The subject invention relates to printing methods and apparatus and to doctor blades and methods for making doctor blades for wiping excess ink from a printing surface of a printing form, especially in photogravure printing.
2. Description of the Prior Art
It is well known that with the different photogravure printing techniques which are employed in the printing art, the ink or dye, which adheres to the etched or engraved wells or depressions of different volume of the flat or cylindrically shaped printing form, is applied under mechanical pressure to the material to be imprinted. The wells or depressions are filled with printing ink and the excess ink is removed by a doctor blade.
With present conventional constructions of rotary photogravure printing machines, also known as rotogravure printing machines, wherein the printing form is a rotating cylinder, there is employed a doctor blade which consists of resilient material. This doctor blade is clamped into a doctor blade holder and extends in the direction of the axis of the rotating printing cylinder. The arrangement is such that the wedge-shaped constructed free end of the doctor blade comes into contact with the rotating cylinder.
The flattened tip of the doctor blade, also known as the doctor blade bevel, which is in contact with the cylinder, insures the removal of the excess ink and is subject to continuous wear. This wear causes an increase in the width of the bevel at the tip of the doctor blade owing to removal of material at the contact region. Due to the increase of the bevel width there occurs tonal increases which constitute some of the most undesirable phenomena.
A width of about 100 .mu.m is about the upper permissible limit of the bevel width. Hence the printer oftentimes must have the doctor blade exchanged after about forty thousand revolutions of the cylinder. This in turn results in frequent downtime of the equipment, and additionally the formation of mackled sheets when placing the machine or equipment again into operation.
The prior art has not been able to overcome these disadvantages and drawbacks, despite a wealth of proposals in the doctor blade art and related fields.
For instance, some known doctor blade designs are of such complexity as to be of little practical value, not only because of extremely high manufacturing costs and complexities, but also because of the presentation of discontinuous or irregular scraping surfaces which bring about streaks and other visible defects in the printed work.
Even with straight edged doctor blades, there always has been a problem of sorts in view of the fact that a very thin blade is difficult to apply evenly across the entire, typically rather long cylinder, and will wear rapidly, while a thick blade would not be supple and flexible enough for some applications.
For instance, an early proposal attempted to improve the wear characteristics of the active front edge, while retaining the suppleness of the overall blade, by providing a doctor blade with a thick front portion as compared to a relatively thin main body of the blade. For a similar effect another proposal provided one or more grooves in the blade body. Another prior-art approach mounted the doctor blade between two spaced plates which left a hollow space between the blade and its holder. The handling of such blades was cumbersome and they were difficult and expensive to manufacture.
In consequence, the continuous or constant thickness doctor blade became and remained the most widely used excess ink wiping means in the printing art under consideration. Where the dimensioning of the doctor blade thickness approached razor blade proportions, a complex backing blade structure, or bulky electromagnetic equipment, or other types of reinforcement was frequently necessary in an endeavor to provide a uniform application of the doctor blade to the associated cylinder.
This generally discouraged a "razor blade approach" to the doctor blade art, inasmuch as the removal of facial hair involves objectives and environments that are alien to the intaglio and photogravure printing art and its requirements. As an exception, one could mention a prior art type of doctor blade which, in a sense, resembles a very modern type of razor blade characterized by a uniform blade thickness having an inclined wedge surface. However, that is where the comparison ends, since doctor blades of the latter type had to be provided with a run-in blade bevel having an optimum size for an achievement of the best printing quality. This led to the above mentioned disadvantages including an increasing deterioration of tonal quality as a function of blade wear.
Tired of high expense and bother entailed by the rapid blade bevel wear of the conventional wedge-tip doctor blade, many printers have resigned themselves to not even attempting to form and maintain a blade bevel at the inclined wedge surface. Instead, they simply turn the doctor blade by 180.degree. about its longitudinal axis and use the wedge surface itself to wipe excess ink from the printing surface. Other printers do this out of a similar frustation when the blade bevel has become worn. The result is the horrible color prints one can see in the magazine portions of many Sunday papers and in sundry other widely disseminated pictorial materials.